Heretofore, rock blasting for reducing ground vibration has been undertaken by employing various methods known as deck charge, air-decking, pre-splitting and so forth. In the deck charge, explosive cartridges and stemmings are alternately charged along the borehole. Each explosive charge in the borehole is provided with a detonator and the explosives are detonated by delay blasting to reduce ground vibration. This method is costly since it requires a large number of detonators in order to be fitted with the charged explosives. Further, this method is less effective in reducing ground vibration since the same amount of explosives must be used in each charge layer.
In the air-decking method, explosive and detonator are charged at the lower part of the borehole to leave space above the explosive charge, and a plug is used to retain the stemming material at the borehole collar. The space between the top of the explosive charge and the bottom of the plug is the air deck used in blasting. The air deck space is rapidly occupied by the detonation products. The detonation products move repeatedly in the chamber, impacting the borehole wall until an equilibrium state in pressure is reached. A significant amount of potential energy retained in the detonation products is released and imparted to the rock mass, which forms a secondary loading wave in addition to the primary loading wave generated by the detonation of the explosive charge. When the air deck is longer than a predetermined length, the volume of the fragmented rock mass is increased. In addition, the average fragment size gets smaller and the size distribution becomes more uniform. Numerical modeling of the effects of air decking is described in the article of Liqing Liu and P. D. Katsabanis in journal Rock Fragmentation by blasting, Mohanty (ed.) 1996 Balkema, Rotterdam, pp.319-330.
In addition, there is indication that the air-decking may reduce ground vibration from ANFO shots to a certain degree. In a field experiment from an article "Ground Vibration from Single-hole Cast Blasts" of Xiaoning Yang, Brian W. Stump and John D. Smith, The journal of Explosives Engineering, March/April 1999, pp. 36-41, slightly reduced ground vibration level (15%) from air-decked shot loaded with ANFO was reported.
However, when the air-decking is adopted in the open large scale blast site, positioning the plug in every borehole at a uniform depth is difficult since the plug is plastic and apt to be moving in the borehole. This results in different length of the air deck in each borehole, thereby degrading the above-mentioned air deck effect. Further, it is difficult to secure constant quantity of air in every air deck since the borehole diameter becomes narrow as drilling bits wear out by use. This also affects the air deck effect.
Another method of producing substantially reduced vibration involves arranging explosive cartridges in a borehole such that they are connected serially by a detonating fuse at spaced intervals to form air space between charged explosives. This method, commonly referred to as pre-splitting, requires use of a large amount of explosive which, upon detonation, produce considerable noise of the blast.
Therefore, the above-mentioned prior art blasting methods have problems in that they are still ineffective either in obtaining quantitative air decking or reducing ground vibration and noise of the blast.